A gas turbine engine is typically mounted below an aircraft wing or within an aircraft tail section by a pylon. The engine is typically mounted at its forward end, an intermediate section, and its aft end for transmitting loads to the pylon. The loads typically include vertical loads such as the weight of the engine itself, axial loads due to the thrust generated by the engine, side loads such those due to wind buffeting, and roll loads or torques due to rotary operation of the engine. The mounts must also accommodate both axial and radial thermal expansion and contraction of the engine relative to the supporting pylon.
Both the forward and aft mounts are typically provided for carrying in-plane loads to the pylon, which in-plane loads are those occurring in a single axial plane extending perpendicularly outwardly from the longitudinal axis of the engine and include vertical and horizontal loads and rotary torque or moments. The thrust mount is provided for transferring the axially directed thrust loads from the engine to the pylon which are tension loads during forward propulsion of the aircraft, and compression loads which occur during the use of the engine's thrust reverser during braking of the aircraft upon landing.
An exemplary conventional thrust mount includes a pair of circumferentially spaced apart elongate thrust links pivotally joined at forward ends thereof to a conventional fan frame, and at opposite, aft ends thereof are pivotally joined to a lever sometimes referred to as a whiffle tree. The two thrust links are pivotally joined to opposite ends of the lever, and the center of the lever is pivotally joined to a platform fixedly joined to the pylon. The several thrust link pivotal joints include conventionally known spherical bearings, also known as uniballs, which allow slight rotation of the thrust links in three orthogonal planes relative to the fan frame and the lever. And, the lever center joint includes a pin through a bushing for single plane rotation.
During forward propulsion of the aircraft, forward axial thrust is carried through the thrust links, which undergo tension, and in turn through the center of the lever to the platform and pylon. The lever provides for a slight rotation about its center to equilize axial load between the two thrust links. When the thrust reverser is deployed, an aft directed thrust force is carried through the thrust links, which undergo compression, and in turn through the center of the lever and the platform to the pylon.
The thrust mount requires failsafe operation in the event of failure of any of its components to provide an alternate loadpath between the fan frame and the pylon to ensure that the thrust loads may be suitably carried. In one exemplary design, the lever includes two aft failsafe extensions at the respective ends of the lever which are positioned in respective failsafe clevises extending forwardly from the platform. Conventional failsafe shear bolts or pins extend through these failsafe clevises and through an enlarged aperture in the lever failsafe extensions. The extension apertures are predeterminedly larger in diameter than the outer diameter of the failsafe pins to provide clearance gaps so that the lever may rotate about its center up to a predetermined angular rotation for allowing normal deviations of the lever during normal expected operation of the engine in the aircraft. However, in the event of a failure of a thrust link, or lever, or one of their pivotal joints, which failure causes the failsafe extension to contact the failsafe pin during operation, thrust loads will then be carried therethrough and through the failsafe clevises to the platform and in turn to the pylon for providing the alternate loadpath.
For example, the joint at the center of the lever may develop a crack which severs the lever in half separating it completely from the platform. In such a failure, the thrust loads will be carried through the alternate paths from the thrust links through the lever ends and in turn through the lever failsafe extensions, failsafe pins, and platform failsafe clevises to the platform and to the pylon.
It is desirable to provide a loadpath from the fan frame to the pylon which is as straight as possible and aligned coaxially with the centerline axis of each thrust link to prevent undesirable bending loads and stresses which would require a larger and heavier thrust mount. However, in a failure of one thrust link loadpath not involving the lever center joint, axial thrust loads will be transmitted through the other thrust link and through both the center of the lever to the platform and the lever failsafe extension to the platform. Therefore, a compromise must be made in the configuration of the respective loadpaths since such loadpaths are necessarily different from each other, and transmitting tension or compression loads without bending in the loadpath is not possible under all circumstances.
Furthermore, since the several joints between the thrust links, lever, lever failsafe extensions and their clevises necessarily have clearances and slight rotational capability in various planes due to the spherical bearings and gaps utilized, unstable operation of the assembly may occur during reverse thrust operation in a failure event. For example, if the lever were to be separated in half through its center joint as above described, each side of the lever would operate independently of the other side in a three-hinged chain of the thrust link joined to the fan frame and lever at first and second hinges, and the lever failsafe extension joined to the platform failsafe clevis at a third hinge. Under tension through the thrust link in forward propulsion of the aircraft, the respective three-hinged loadpath through the thrust links would merely elongate slightly as in a chain in tension but nevertheless with the ability for restraining tension loads being substantially unimpaired. However, if compressive thrust loads are carried by the thrust links during reverse thrust operation in such a failure, the two joints or hinges created between the thrust link and the lever end and the lever failsafe extension and the platform failsafe clevis will allow rotation in one or more planes. This leads to misalignment of the components and the inability to transmit compressive thrust loads which is like attempting to transmit compressive loads through the chain in the above analogy.
Accordingly, in view of this potential instability in a failure under a reverse thrust condition, the lever is required to be designed as a monolithic or non-failsafe item which increases its strength and correspondingly the size and weight of the thrust mount. However, the space available for the entire thrust mount and requirements for maximum weight of the thrust mount for a particular design place limits on the design of the thrust mount. In one present application, the use of such a conventional thrust mount is not desirable because space and weight limitations would be exceeded for a given maximum thrust capability requirement.